1. Field of Invention
The present invention relates to an error detecting method and apparatus, more particularly to an error detecting method and apparatus for detecting errors before C1 decoding procedure.
2. Description of Related Art
Generally, before recognizing data stored in an optical disk, the front-end circuit of the optical disk drive is employed to transform an analog RF (radio frequency) signal read from the optical disk into series of digital data channel bits. These digital data channel bits are subsequently modulated by a modulating unit. For example, an EFM unit performs an eight-to-fourteen bit modulation (EFM) for data accessed from an optical disk such as a CD-ROM, while an eight-to-sixteen bit modulation is executed for modulating data read from those DVD-type optical disks. In the following paragraphs, a CD-ROM is chosen as an example for the sake of explanations.
FIG. 1 is a schematic diagram showing an error detecting apparatus in a conventionally optical disk drive. In general, an optical disk drive includes an EFM unit 10 for modulating each data having 14 data channel bits (“14-bit data” as for short hereinafter) obtained from the front-end circuit into an 8-bit data. The EFM unit 10 refers to a look-up table 11 defining 256 relationships between 14-bit and 8-bit data mapping during the EFM modulation. Table 1 shows a portion of the EFM look-up table, which can be constructed in the optical disk drive.
TABLE 1EFMChannel BitsData Bits001001000100000000000001100001000000000000000121001000010000000000010310001000100000000000114010001000000000000010050000010001000000000101. . .. . .. . .255 0010000001001011111111
Since there are 28=256 and 214=16384 different combinations for the 8-bit and 14-bit data, respectively, the combinations of the 8-bit data is considerably lower than that of the 14-bit one. Obviously, only 256 mappings between the 14-bit and 8-bit data are authorized and existed in the EFM look-up table, and the rest combinations of the 14-bit data non-defined in the EFM look-up table are all illegal mappings. Sometimes, the modulated 8-bit data may be erroneous when the combination of the 14-bit data obtained from the front-end circuit becomes a non-defined one of the EFM look-up table. Related error detection and correction are therefore activated for error recovery purpose. At present, cross-interleave Reed-Solomon code (CIRC) is the most commonly used approach for detecting and correcting errors of data accessed from an optical disk.
Please refer to FIG. 1. Each of the 14-bit data obtained from the front-end circuit is modulated to 8-bit data in the EFM unit 10 whenever the 14-bit data is legal. A one symbol delay unit 12 in FIG. 1 receives and arranges the modulated 8-bit data subsequently and then outputs a codeword composed of 32-byte data, while a C1 decoder 14 receives the codeword and performs a C1 decoding procedure. In the conventional CIRC decoding method, this C1 decoder at most provides a capacity for decoding two errors. That is, a conventional C1 decoder is capable of detecting positions of two errors, and evaluating two correct values for these two detected errors. In other words, the C1 decoder 14 can decode only up to two errors even there are more than two errors in a codeword because the conventional C1 decoder has to share capability of detecting where the errors allocate. For the convenience of identifying a codeword might have error embedded inside, the C1 decoder 14 tags an erasure bit for each modulated 8-bit data. For example, a “0” erasure bit tagged at the modulated 8-bit data after the C1 decoding procedure represents this modulated 8-bit data as “correct,” and a “1” erasure bit tagged at the modulated 8-bit data after the C1 decoding procedure represents the modulated 8-bit data “may contain an error.”
After that, the C1 decoder 14 delivers 28 erasure bits attached to 28 bytes of data to an interleaving unit 16 for data interleaving. These 28 erasure bits and the 28 bytes of interleaved data are then directed to a C2 decoder 18 to continue a C2 decoding procedure. A conventional C2 decoder 18 cannot decode data having more than four errors because the conventional C2 decoder at most provides a capacity for decoding four errors. The C2 decoder 18 also tags an erasure bit for each interleaved 8-bit data. Tagging this erasure bit is for easily identifying whether the interleaved 8-bit data might be erroneous after the C2 decoding procedure. For example, a “0” erasure bit tagged at the interleaved 8-bit data after the C2 decoding procedure represents the interleaved 8-bit one is accurate, while a “1” erasure bit tagged for the interleaved 8-bit data after the C2 decoding procedure represents the interleaved 8-bit data might be erroneous. Thereafter, the C2 decoder 18 delivers 24 erasure bits attached to 24-byte data into a two symbol delay unit 20. This CIRC decoding approach mainly used in the aforementioned C1 and C2 decoders is to detect error positions and to evaluate correct values for correcting purpose.
In conclusion, there are many constraints in the conventional CIRC decoding method for the optical disk drives because there are restrictions on the error decoding capacities of the decoders, for example, the conventional C1 decoder is limited to decode two errors. Therefore, there is a need to disclose a method for overcoming the aforementioned disadvantage such that an optical disk capable of operating under high speeds for accessing large data-storage media may operate under sufficient error decoding capacity.